Simulating the Milky Way

(Kobayashi)

Chemical Enrichment

Different elements are produced from
different types of supernovae with different timescales, with the
consequence that elemental abundance ratios evolve inside a galaxy.
Using chemical evolution models, the time evolution of elements have
been well reproduced in the solar neighbourhood with metal-dependent
nucleosynthesis yields of 1) core-collapse supernovae for massive stars
(Kobayashi et al. 2006), 2) thermonuclear supernovae from low-mass
binary stars with the metallicity effect (Kobayashi et al. 1998), and
low-mass AGB stars (Kobayashi et al. 2011).

Evolution of elemental abundances relative to iron [X/Fe] against [Fe/H]
in the solar neighbourhood. The dots are for observations of stars
taken from various literatures, and the lines are for our one-zone
chemical evolution model.

High-resolution multi-object spectrographs such as GAIA-ESO survey, APOGEE, HERMES, WEAVE,
in which we are participating, will deliver elemental abundances for
million of stars in the Local Group. In order to understand the origin
of the scatter, we need a realistic model.

Chemodynamical simulations

We study the formation and
evolutionary histories of Milky Way-type galaxies using the chemo-dynamical
simulations described by Kobayashi & Nakasato (2011). These
incorporate Smoothed Particle Hydrodynamics (SPH) to describe
hydrodynamics, and direct summation with the GRAPE system or tree method
describes gravity. The relevant physical processes such as radiative
cooling, star formation, supernovae feedback, and chemical enrichment
are all included. This enables these simulations to be compared with
observed elemental abundances of stars in various locations - bulge,
disk, halo, and satellite galaxies.
In the simulations, the bulge have formed by assembly of gas-rich
subgalaxies. The disk forms inside-out with a longer timescale. A half
of the thick disk stars come from the minor merging of satellite galaxies.

B-band Luminosity Map of our simulated Milky Way-type galaxy at present
for the face-on (left panel) and edge-on (right panel) projections. Each
panel is 20 kpc on a side. Watch the movie on this link.

Galactic Archaeology

Using our simulations, we have predicted
the time/redshift evolution of the spatial distribution of elements from
Lithium to Zinc (A=30). This will be extended up to Uranium (A=92)
using new AGB yields and r-process calculations (Kobayashi et al. 2015).
By comparing with observations, we aim to untangle the star formation
and chemical enrichment histories of the Milky Way Galaxy.

[X/Fe]-[Fe/H] relations in the solar neighbourhood at z=0. The contours
show the frequency distribution of stars in the simulated galaxies,
where red is for the highest frequency. The white dots are for
observations taken from various published studies.

This quest is fundamentally tied to the mystery of the origin of the
elements, which has a clear connection with the origins of life. Our
human body comes 90% from stellar dust. But what kind of stars were they? How did
they explode?